Method for impregnating wood

ABSTRACT

METHOD FOR IMPREGNATING WOOD WITH A TREATING LIQUID SUCH AS A PRESERVATIVE, A FIRE RETARDANT OR THE LIKE, WHEREIN THE WOOD IS IMMERSED IN THE TREATING LIQUID INSIDE A PRESSURE VESSEL. THE PRESSURE IN THE VESSEL IS BROUGHT UP TO OPERATING PRESSURE WITHIN A SHORT TIME, ON THE ORDER OF TEN MINUTES OR LESS, AND THE CONTENTS OF THE VESSEL ARE THEN SUBJECTED TO THE ACTION OF A PULSATING PUMP WHICH PROVIDES SINUSODIAL PRESSURE PULSES WITHIN THE VESSEL HAVING A POSITIVE PEAK TO NEGATIVE PEAK AMPLITUDE OF FROM 1 TO 1500 POUNDS PER SQUARE INCH. THESE PRESSURE PLUSE ARE MODULATED BY THE ACTION OF A PRESSURE RELIEF MEANS ASSOCIATED WITH THE PRESSURE VESSEL, WHEREBY THE RESULTANT PRESSURE ACTING ON THE WOOD BEING IMPREGNATED CONSISTS OF A SERIES OF PRESSURE PULSES MODULATED IN AMPLITUDE AT A RELATIVELY SLOW RATE. THE TREATING LIQUID WHICH IS DISCHARGED THROUGH THE PRESSURE RELIEF MEANS IN A PULSATING FASHION IS THEN RECYCLED BACK TO THE PULSATING PUMP SO THAT THERE IS A SUBSTANTIALLY CONTINUOUS FLOW OF TREATING LIQUID FROM THE PUMP, THROUGH THE PRESSURE VESSEL, THROUGH THE PRESSURE RELIEF MEANS, AND FINALLY BACK INTO THE INLET END OF THE PUMP.   D R A W I N G

y 1972 J. H. BARNETT, JR 3,677,305

METHOD FOR IMPREGNATING WOOD FilOd March 4, 1970 &

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United States Patent O1 ice U.S. Cl. 117-102 A 4 Claims ABSTRACT OF THE DISCLOSURE Method for impregnating wood with a treating liquid such as a preservative, a fire retardant or the like, wherein the wood is immersed in the treating liquid inside a pressure vessel. The pressure in the vessel is brought up to operating pressure within a short time, on the order of ten minutes or less, and the contents of the vessel are then subjected to the action of a pulsating pump which provides sinusodial pressure pulses within the vessel having a positive peak to negative peak amplitude of from 1 to 1500 pounds per square inch. These pressure pulses are modulated by the action of a pressure relief means associated with the pressure vessel, whereby the resultant pressure acting on the wood being impregnated consists of a series of pressure pulses modulated in amplitude at a relatively slow rate. The treating liquid which is discharged through the pressure relief means in a pulsating fashion is then recycled back to the pulsating pump so that there is a substantially continuous flow of treating liquid from the pump, through the pressure vessel, through the pressure relief means, and finally back into the inlet end of the pump.

BACKGROUND OF THE INVENTION Field of the invention This invention is in the field of impregnating wood with modifying liquids such as preservatives or fire retardant agents. It makes use of sharp peaked pressure pulses applied repetitively in modulated amplitude to provide variable pressure peaks above and below the ambient pressure which is maintained in the pressure vessel. The equipment employed includes a pulsating pump operating into a pressure vessel which is equipped with a pressure relief means such as a spring-biased pressure relief valve.

Description of the prior art Non-refractory species of wood are commonly impregnated with preservative, fire retardant and other modifying liquids. Typically, such installations include pressure vessels supplied by pumps which gradually bring up the pressure in the pressure vessel and maintain it over extended periods of time. This type of treatment is described by MacLean in the U.S. Department of Agriculture Forest Service Agriculture Handbook, No. 40, 1952, page 88, entitled Preservative Treatment of Wood by Pressure Methods.

The prior art of wood treatment goes back more than one hundred years and includes various procedures for conditioning the wood to make it more receptive to the subsequently applied treating agent. For example, in Rueping U.S. Pat. No. 709,799, there is disclosed a method wherein a charge is subjected to compressed air at sufiicient pressure and for a sufiicient time, depending on the species of the wood, to control the retention of preservative or fire retardant liquid to specified limits. Lowry U.S. Pat. No. 831,540 provides for the use of ambient atmospheric air pressure in the charges. British Pat. No. 7,731 of 1838 to Bethell suggested that the charge be subjected to vacuum for a specific period of time, followed 3,677,805 Patented July 18, 1972 by introduction of the treating liquid to displace the vacuum. The Boulton method described in U.S. Pat. No. 247,602 provided for rapid moisture removal from wet wood by boiling under either atmospheric pressure or under vacuum, with oil-type preservatives. Hudson in U.S. Pats. Nos. 2,273,059; 2,435,218; and, 2,435,219 suggested a method for exposing green wood to vapors of a high flash solvent naphtha at an elevated temperature. When the vapor was condensed, water separated and was discharged from the system. The vapor drying cycle was followed by a one to two hour vacuum treatment for recovery of the solvent naphtha from the wood, and then preservative liquid was introduced to displace the vacuum prior to pressure pumping.

The prior art also includes disclosures of the use of saw toothed waveforms superimposed on the ambient pressure, as shown in Page and Reed, U.S. Pat. No. 3,467,546. Bodine in his U.S. Pat. No. 3,410,532 described a high Q acoustical circuit employing a massive stationary element and a massive vibratory element, the latter being activated into vibratory improvement by a resonant elastic system.

SUMMARY OF THE INVENTION The present invention provides an improved method and apparatus for economically and expeditiously impregnating wood with a treating liquid. In the operation of the system in the present invention, the Wood is first immersed in a body of the treating liquid located within an enclosed pressure vessel, and quickly brought up to an impregnating pressure within the range of about 20 to 700 pounds per square inch gauge within 10 minutes, and preferably within 3 minutes. A pulsating pump is provided in fluid communication with the interior of the vessel and operates to generate pressure pulses in the vessel which have a positive peak to negative peak amplitude in the range from 1 to 1500 pounds per square inch. These pulses have a frequency preferably within the range from 0.1 to 10,000 hertz. A pressure relief means such as a springbiased pressure relief valve is also included in the pressure vessel, the pressure relief valve having a sufficient capacity to pass at least 10% of the rated capacity of the pulsating pump during operation. This pressure relief valve has a natural period of vibration depending upon the mechanical constants embodied within it, so that it serves to discharge quantities of liquid from the pressure vessel which vary instantaneously depending upon the relative position of the plug within the pressure relief valve. This mechanical action serves to modulate the pressure pulses which are being generated by the pulsating pump at a relatively low frequency on the order of 0.01 to 10 hertz. The pulsating discharge passing through the pressure relief valve is then recirculated back to the inlet side of the pump. Thus, a substantially continuous movement of liquid exits from the pump, through the pressure vessel, through the pressure relief means and back into the pump which serves to keep the pump operating temperature down, and also serves to provide more efficient heat transfer for the treating liquid as it flows by the heating element located in the pressure vessel.

Other objects, features and advantages of the invention will be readily apparent from the following description of certain preferred embodiments thereof, taken in conjunction with the accompanying drawings, although variations and modifications may be effected without departing from the spirit and scope of the novel concepts of the disclosure, and in which:

FIG. 1 is a somewhat schematic view of an installation for practicing the present invention; and

FIG. 2 is a fragmentary cross-sectional view illustrating the spring-loaded pressure relief valve more completely.

3 DESCRIPTION OF THE PREFERRED EMBODIMENTS The coaction between the pulsating pressure pump and the pressure relief means provides a modulated wave train which appears to excite the wood and penetrant for the purpose of causing occluded air in the wood structure to be dispersed as extremely fine bubbles and to escape, thus maintaining the pit membranes in an unaspirated or open condition. The excitation, when properly applied, causes the wood microstructure to behave as a standard porous substrate, free from the restraining effects of pit membranes which act as check valves, and free from blockage by surface tension generated menisci in the wood pores. Air which escapes from the wood during excitation rises to the top of the pressure vessel and is vented through the pressure relief valve.

Where the size of the pressure vessel is large and economy dictates the use of pumping rates at a relatively low value on the order of 0.1 gallon of liquid per minute, per cubic foot of wood normally charged, in order to get adequate excitation levels for refractory wood species, an auxiliary pressure pulse transducer can be used. In smaller pressure vessels where it is economical to pump the liquid at rates at or near 0.5 gallon per minute, per cubic foot of wood normally charged, adequate excitation can be obtained for all except the most refractory species of woods without the use of such auxiliary transducer. By cubic foot of wood normally charged is meant the usual volume occupied by the wood in a pressure vessel of a given size. Relating this to vessel capacity, the capacity of the pump should be between 0.045 and 0.225 gallon per minute, per cubic foot of volume of pressure vessel.

The instantaneous peaks of positive and negative pressure required to excite charge of wood properly in the pressure vessel are variable from specie to specie of wood. Generally, as the pore openings in the wood decrease in diameter, the instantaneous peak pressures required increase in proportion to the fourth power of the pore radii. The less refractory woods appear to have mean port opening diameters on the order of 3.5 microns or so, and to obtain adequate excitation in them requires instantaneous peak pressures in the pressure pulse trains to be from 5.0 to 100 pounds per square inch amplitude between the positive and negative peaks.

Woods of the so-called refractory type that are not completely blocked by resinous deposits or tumorous growths in the cells appear to require much higher instantaneous peak pressures, on the order of 1000 to 1500 pounds per square inch amplitude between the positive and negative peaks.

In the preferred form of the present invention, the pulsating pressure pump can be of the centrifugal type, either single stage or multi-stage, a positive displacement pump, or a piston pump. In any event, it should have characteristics such that it produces pressure pulses having a positive peak to negative peak amplitude of from 1 to 1500 pounds per square inch, the specifice values being dependent upon the specie of Wood being treated, as previously discussed. The capacity of the pump should be sufficient to pump between 0.1 to 0.5 gallon of liquid per minute, per cubic foot of wood normally charged.

The pump employed should have sufficient volume capacity, when pumping, to maintain an operating pressure in the vessel in the range from 20 to 700 pounds per square inch. In operation, at least of the rated volume capacity of the pump at operating pressure is bypassed through the pressure relief valve for the purpose of providing cooling liquid for the pressure pump and providing adequate heat exchange for the liquid circulating through the pressure vessel.

Circulation of the contents of the pressure vessel provides more rapid heating of the liquid by steam circulating in the heating coils, present in the vessel, and provides faster heating of the pieces of wood in the charge to approach the temperature of the treating liquid. When pieces of wood in the charge are cold as, for example, by exposure to low temperatures prior to charging into the pressure vessel, the temperature of the treating liquid at the wood-liquid interface is quickly reduced, causing the viscosity of the liquid to increase, with resulting reduction in penetration rate. Circulation of hot liquid, and maintenance of the liquid at a proper temperature serves to sweep the thin, cooled layers of liquid from the wood surface and provides hot liquid layers in these areas, resulting in more rapid penetration of the wood by the treating liquid.

Turning now to the drawings, reference numeral 10 indicates generally a pressure vessel to which an access door 11 is suitably bolted and sealed to permit the charge of wood to be introduced into and removed from the pressure vessel 10. Heating coils 12 are disposed in the pressure vessel 10 and steam or other heated fluid is circulated through these coils to heat up the treating liquid for improved penetration. A pulsating pressure source such as a centrifugal pump 13 driven by a motor 14 is coupled in direct fluid contact with the interior of the pressure vessel 10 by means of a nozzle 15 extending into a fitting 16 which is secured to the vessel. Spacer lugs 17 are positioned about the nozzle 15 to provide greater strength.

An input line 18 connected to the pump 13 is fed by means of a conduit 19 through a valve 20 from a suitable reservoir (not shown) containing the treating liquid. The venting of the contents of the pressure vessel 10 to the atmosphere during filling or removal of the treating liquid from the vessel is accomplished by providing a venting line 21 in which there is located a valve 22, the valve 22 being closed except when the pressure vessel is either filling or having its contents discharged.

For the purpose of withdrawing liquid from the pressure vessel, there is provided a discharge line 23 controlled by a valve 24, the discharge being recirculated to the reservoir for the treating liquid.

The assembly also includes a spring-loaded pressure relief valve generally indicated at reference numeral 25 in FIGS. 1 and 2. Preferably, this valve is located at the end of the pressure vessel opposite to the pump end and usually within about 10% of the length of the vessel from the end. As shown, the valve 25 includes a plug 26 which is reciprocable in the housing and is under the control of a spring 27. A preset biasing pressure is applied to the plug 26 by an adjusting mechanism generally indicated at numeral 28. The dimensions of the valve 25 should be such that it will permit at least 10% of the pumps rated capacity for flow through the valve into a line 29 which eventually merges with the inlet conduit 28 and recirculates the liquid back to the input side of the pump.

Undamped pressure relief valves of the type here de' scribed have a characteristic oscillating frequency dependent upon the mechanical parameters present in the valve. For the purpose of the present invention, it is important that such oscillating frequency be substantially less than the frequency of the pulses generated by the pump 13, and should normally be in the range from 0.01 to 10 hertz. In the system described, the pressure vessel 10 is large and its shell has some ability to stretch. While the liquid treating agent is substantially non-compressible, the wood in the charge is compressible. The wood contains a volume of air at its center. The combination of the stretchability of the large area of the pressure vessel walls and the compressibility of the wood results in a highly resilient system where a pressure pulse which is applied yields a resounding bounce that causes the whole vessel and its contents to oscillate at varying frequencies. The pressure vessel under load is a broadly resonant apparatus which responds to the frequencies that the oscillating relief valve and the pump pulses create.

The following specific example illustrates some of the results achieved when operating according to the present invention.

EXAMPLE Gross impregnation, 16.95 lbs. per cu. ft. Net retention, 10.74 lbs. per cu. ft.

Minutes: Percent gross injected 2 21 4 v 42 6 56 8 72 1O 84 12 90 14 95 I6 98 18 100 NOTE: Assay for coal tar creosote in the /2-2" zone, average, 10.27 lbs. per cu. ft.

To secure the same extent of impregnation would require 120 to 180 minutes of pumping time if the material had been treated conventionally in the same type of equipment.

I claim as my invention:

1. The method of impregnating wood with a treating liquid which comprises immersing said wood in the treating liquid in a confined pressure vessel, bringing the pressure in said vessel to an impregnating pressure within no more than about ten minutes, subjecting the immersed wood to sinusoidal pressure pulses having positive peaks above the ambient pressure in the vessel and negative peaks below said ambient pressure, the positive peak to negative peak amplitude of said pulses being in the range from 1 to 1,500 pounds per square inch, and cyclically discharging liquid under positive pressure from said pressure vessel at a rate less than the pulse repetition frequency of said pressure pulses, and in the range from about 0.01 to 10 hertz.

2. The method of claim 1 in which the pressure pulses occur at a frequency of from 0.1 to 10,000 hertz.

3. The method of claim 1 in which said impregnating pressure is in the range from about 20 to 700 pounds per square inch gauge.

4. The method of claim 1 in which the liquid withdrawn from the pressure vessel is repumped back into said pressure vessel.

References Cited UNITED STATES PATENTS 3,467,546 9/ 1969 Page et a1. 117--116 2,786,784 3/1957 Henri-ksson 117-116 3,410,532 11/1968 Bodine 259-4 3,318,725 5/1967 Bryan 117147 WILLIAM D. MARTIN, Primary Examiner M. SOFOCLEOUS, Assistant Examiner US. Cl. X.R. 

